Typically, a rotating anode x-ray tube is made up of an evacuated envelope in which a cathode and an anode are positioned. A heating current is provided to the cathode and a large potential is created between the anode and the cathode in order to accelerate the electrons from the cathode to the anode. The anode is a rotating disk and the target area on the anode is typically a small area of the anode which is located towards the circumference of the disk.
The anode disk is supported by a shaft which in turn is supported on a bearing. The shaft is rotated at a high speed by means of electro-magnetic induction from a series of stator windings which are located outside of the evacuated envelope. The stator windings act on a cylindrical armature or sleeve which is fixed to the shaft. The bearing is positioned in the envelope between the shaft and the armature to allow the shaft and the armature to rotate, thereby rotating the disk. Typically, the inner bearing races are part of the shaft while the outer bearing races are part of a sleeve which is fixed to the envelope. Roller bodies are positioned in the races.
One of the problems associated with rotating anode x-ray tubes is that a great deal of heat is generated inside the tube which can have a deleterious effect on the bearing elements. Typically, in order to address the temperature problem, various cooling arrangements have been devised such as the ones shown in U.S. Pat. Nos. 6,445,770 and 6,445,769.
There can be a significant temperature difference between the outer races and the inner races during the starting up process until the temperature within the tube has stabilized. This temperature difference can potentially cause the outer races to grow both radially and axially much faster than the inner races. Because of this difference in thermal expansion, a large amount of internal radial clearance must be built into the bearing, causing the bearing to be noisy and to greatly reduce the life of the bearing. Typically, high temperature hardened materials are used for the outer bearing. Those materials can be expensive and thereby increase the cost associated with the bearing.